The University of Florida Theoretical Astrophysics Seminar is held on Wednesdays at 1:45pm in Room 2165 of the New Physics Building, unless otherwise noted.

Spring 2019 Schedule

January 18: Kunal Mooley (Caltech)

Title: The First EM Counterpart of a Gravitational Wave Event


On 17 Aug 2017, the LIGO and Virgo observatories announced the detection of the binary neutron star merger GW170817. The gravitational wave signal was accompanied by a low-luminosity burst of gamma-rays. Within 12 hours of the merger an optical counterpart, powered by r-process nucleosynthesis of ~0.05 M_sun of neutron-rich ejecta, was discovered and localized to a spiral galaxy NGC 4993 at a distance of 40 Mpc. X-ray and radio signals, consistent with the synchrotron afterglow, were detected 1-2 weeks after the merger. The radio afterglow of GW170817 gave key insights into the energetics, morphology of the ejecta and the circum-merger environment. Radio interferometric observations have confirmed that an energetic and narrowly-collimated jet interacted with the kilonova ejecta to produce a wide angle outflow (cocoon) that dominated the radio afterglow at early times. The jet successfully penetrated the ejecta and dominated the emission at late times. Radio VLBI observations together with state-of-the-art hydrodynamical simulations suggest that the jet had an opening angle of less than 5 degrees and was observed from a viewing angle of about 20 degrees. The strong constraint on the geometry allows us to precisely measure the rate of expansion of the nearby Universe (the Hubble's constant through the "standard siren" technique). The radio afterglow of GW170817 has provided one of the strongest observational evidence, so far, that binary neutron star mergers produce short-hard GRBs. Although a unique event, GW170817 represents only an initial exploration of a rich scientific landscape populated by the stellar evolution, explosion and eventual merger of massive binary systems. The study of the EM counterparts (especially radio afterglows) of neutron star mergers detected by LIGO/Virgo over the coming years will be able to address many unsolved questions pertaining to central engines, fundamental physics, cosmology, and binary stellar evolution.

Feb 6: No Seminar -- BASS Workshop

Feb 8 (4pm, NPB 1002): Special Department Colloquium: Gabriela González (LSU)

Title: Gravitational Waves Astronomy

February 27: Luke Kelley (Northwestern)

Title: Electromagnetic signatures of Binary Supermassive Black Holes


LIGO has now detected almost a dozen gravitational wave (GW) signatures from merger stellar-mass black holes. Supermassive black holes (SMBHs) also emit GW, at nanohertz frequencies expected to be detected by pulsar timing arrays in the next decade. In the mean time, the number of candidate SMBH-binary systems identified in electromagnetic surveys is rapidly growing, although none have yet been confirmed. In this talk, I will discuss our results modeling the expected properties and detection rates of SMBH binaries as photometrically-variable active galactic nuclei (AGN). While many of the existing variability candidates are likely false positives, we find that numerous true binaries should exist in the data. While our results hint at what candidate parameters are most likely real sources, I will also briefly discuss what the contamination tells us about accretion processes in single AGN. Finally, I will present recent work examining the signatures and detection prospects of spectroscopic binary AGN, and those from VLBI and micro-lensing surveys. The multi-messenger landscape of SMBH binary observations in both EM and GW spectra offer an exciting future of discoveries in the physics of galactic nuclei, SMBH feeding, and the dynamics of galaxy mergers and many-body systems.

March 6: No Seminar - Spring Break

March 13: Jim Fry (UF)

Title: Large Scale Structure in the Halo Model


Today we have a richness of CMB, weak lensing and now perhaps 21cm data, but at the beginning of the subject the large scale structure of the universe was what was inferred from the distribution of galaxies on the sky. The advantage is that galaxies are visible to the eye, but any catalog of galaxies is subject to selection errors, the distribution of even the dark matter is complicated by nonlinearity in evolution, and galaxies are biased tracers of matter. The so-called halo model has provided a phenomenological understanding that seems to work well for explaining galaxy clustering at the correlation function or power spectrum level, but I show that it also works well for a variety of questions it was in no way designed to answer.

March 20: Vera Gluscevic (IAS->UF)

Title: Dark Matter Throughout Cosmic History


Ours is a dark Universe: its astrophysical systems are but a minor addition to dark matter, whose abundance six times outweighs all other particles in the Universe. The physical nature of dark matter is a pressing question, whose answer will likely lead to discovery of new fundamental physics. I will discuss how the coming decade of cosmological observations will open up a wide new window into the physics of dark matter interactions, complementing terrestrial lab searches. In particular, I will review the status of current searches with data from the Planck satellite and present forecasts for the next-generation observations of the cosmic microwave background radiation. I will then use a recent claim of a potential detection of a dark-matter signal from a low-frequency radio observation of the epoch of Cosmic Dawn, in order to demonstrate the necessity for comprehensive analyses in establishing a discovery. Finally, I will discuss the potential of near-field cosmology enabled by upcoming galaxy surveys for uncovering the fundamental nature of dark matter.

March 27: Cliff Will (UF)

Title: Adventures of a general relativist in an N-body world


In weak-field regimes, the effects of general relativity (GR) are generally tiny, but over long timescales, they can sometimes have surprising consequences. We have been exploring such effects in a variety of N-body systems, where N can be two, three and higher. Some examples include the effects of GR on orbital flips in three-body systems, the effect of GR-N-body interactions on Mercury’s perihelion advance, and the effects of spin on the long-term evolution of very eccentric orbits around spinning black holes.

April 17: Wen-fai Fong (Northwestern)

Title: Neutron Star Mergers Across Cosmic Time


Multi-messenger astronomy coupling gravitational waves and light was born with the detection of the first neutron star merger, GW170817. This discovery signaled a wealth of firsts in physics and astronomy and represents only the tip of the iceberg of the discovery potential we can achieve in multi-messenger astronomy. In this talk, I will focus on two intriguing aspects of this event: the 500-day evolution across the electromagnetic spectrum, which provides unprecedented insight into the launch of relativistic material from these explosions, and the unique host galaxy environment of GW170817. I will also discuss an ongoing deep survey of the environments of short gamma-ray bursts, which serve as cosmological analogues to the local population of mergers visible to current and near-future GW detectors.

Fall 2018 Schedule

September 12: George Privon (UF Astronomy)

Title: Dynamical Models for Galaxy Mergers


Galaxy mergers in the local universe are known to trigger bursts of star formation and the growth of supermassive black holes. The observational evidence is broadly consistent with numerical predictions but detailed comparisons are difficult due to ambiguities in the merger stages of observed systems. I will describe detailed dynamical modeling of observed mergers and the resulting constraints on the encounter geometry and merger stage. I will then show how these models can be used to place observed galaxy properties on a robust merger timeline, facilitating more detailed comparisons with galaxy evolution simulations. Finally, I will discuss prospects for these models as an avenue for constraining galaxy merger rates and the timing of black hole growth in mergers.

September 19: Maria Charisi (Caltech)

Title: Quasars with periodic variability as sub-parsec supermassive black hole binary candidates

Supermassive Black Hole Binaries (SMBHBs) are expected to form frequently in galactic nuclei as a result of galaxy mergers. However, the observational evidence for such systems, especially for binaries at sub-parsec separations, remains sparse. Hydrodynamical simulations predict that SMBHBs produce quasar-like luminosities, with variability periodically modulated at the orbital period of the binary. I will present a systematic search for quasars with periodic variability in the Palomar Transient Factory and the detection of a population of candidate SMBHBs. Moreover, I will also discuss follow-up work to uncover additional evidence for the binary nature of the identified candidates, such as multi-wavelength signatures of relativistic Doppler boost.

September 26: Kyle Kremer (Northwestern Univ.)

Title: How Black Holes Shape Globular Clusters


Recent numerical simulations and observational indications have suggested that globular clusters (GCs) may contain large populations of stellar-mass black holes (BHs). This has important implications to gravitational wave astronomy, as GCs are understood to be efficient factories of merging BH-BH binaries. In this talk, I will examine ways BH populations influence the dynamical evolution and observable properties of GCs and also the discuss the dynamical formation of various types of BH binaries, including both low-mass X-ray binaries and gravitational wave sources that may be detectable by LIGO and LISA. Additionally, I will discuss the formation of accreting white dwarf binaries and their applications to LISA.

October 17: Marcos Santander (Univ. of Alabama)

Title: Unveiling the neutrino sky with IceCube


The IceCube Neutrino Observatory consists of a cubic kilometer of Antarctic ice instrumented with sensitive photodetectors deployed deep beneath the surface of the glacier at the geographic South Pole. In 2013 IceCube announced the detection of a cosmic flux of high-energy neutrinos in the TeV-PeV range. This breakthrough discovery has prompted a wide-ranging observational effort aimed at identifying the sources of the neutrino flux which combines IceCube measurements with observations spanning the entire electromagnetic spectrum. The detection of these sources would represent a major step forward for high-energy astrophysics that could reveal the acceleration sites of cosmic rays and provide a unique probe into the extreme environments in which these particles are produced. Recent multi-messenger campaigns have yielded the first evidence for a distant active galaxy as a high-energy neutrino source. This talk will introduce the IceCube detector, provide a summary of recent highlights from its wide research program concentrating on the search for neutrino sources, and outline how future instrumental developments may help pinpoint additional neutrino counterparts.

October 24: Raffaella Margutti (Northwestern Univ.)

Title: Astronomical Transients in the new era of multi-messenger Astrophysics


New and improved observational facilities are sampling the night sky with unprecedented temporal cadence and sensitivity across the electromagnetic spectrum. This exercise led to the discovery of new types of astronomical transients and revolutionized our understanding of phenomena that we thought we already know. In this talk I will review some very recent developments in the field that resulted from the ability to acquire a true panchromatic view of the most extreme stellar deaths in nature.

November 7: Wolfgang Tichy (Florida Atlantic Univ.)

Title: Gravitational waves from binary neutron star simulations


When compact objects such as neutron stars or black holes occur in binary configurations they can loose energy and spiral toward each other via the emission of gravitational waves. Waves from such inspirals have now been observed for both binary black holes and binary neutron stars. In order to analyze them it is important to have theoretical wave templates. We describe how numerical relativity simulations can be used to construct such templates. We start with a brief introduction to General Relativity and explain what black holes and neutron stars are. Then we explain how we conduct our numerical simulations, and how they can be used to extract interesting information form observed gravitational wave signals. For example, it is possible constrain the equation of state of neutron star matter by comparing gravitational waves from actual detections and numerical simulations. To do so we have to conduct simulations for various star masses, spins and equations of state.

November 14: Jillian Bellovary (Queensborough Community College)

Title: Multimessenger Signatures of Massive Black Holes in Dwarf Galaxies


Recent discoveries of dwarf galaxies hosting active galactic nuclei (AGN) suggest that massive black holes (MBHs) may be a common occurrence in such low-mass systems. We examine the occupation and characteristics of MBHs in dwarf galaxies using the MARVEL-ous Dwarfs sample of high resolution cosmological simulations. We describe in detail the properties of seven dwarfs hosting MBHs, although none of them are AGN at any point during their histories. Approximately 50% of MBHs in the dwarfs are off-center, as a result of being perturbed by a merger with another dwarf galaxy. Our simulations include a subgrid model for dynamical friction; thus these off-center MBHs are a true dynamical consequence of mergers, and not a numerical effect. Since not every MBH in a dwarf is centrally located, some presupposed MBH-MBH mergers may not actually occur, which will affect the gravitational wave signal detected by LISA.

November 21: [Thanksgiving Week: No Seminar]

November 27, 2:00pm: Ann-Marie Madigan (CU Boulder)

[Note unusual date/time; joint w/ High Energy Seminar]

Title: The Importance of Being Eccentric


The dynamics of bodies on eccentric orbits largely determines the evolution of planetary systems and stars near massive black holes. In this talk I will review eccentric dynamics and demonstrate wide-ranging implications such as the orbital clustering of trans-Neptunian objects in the outer solar system (which motivates the planet nine hypothesis), ` double' galactic nuclei, and the tidal disruptions of stars by massive black holes in post-merger galaxies.

November 28: [No Seminar: Academic Jobs Panel]

Other Events

Students may receive credit for attending the Astrophysics Seminar by registering for PHY 6391.